Aluminum melting method using analysis of fumes coming from the furnace

ABSTRACT

The invention concerns an alumnium melting method, which consists in introducing solid aluminium into a furnace ( 2 ), melting the aluminium to form an aluminium bath ( 4 ), detecting variations in carbon monoxide (CO) and/or hydrogen (H2) concentration and the temperature in the fumes ( 6 ) exiting from the furnace, deducing therefrom the formation of aluminium oxide at the surface of the alumnium bath ( 4 ), regulating the melting process on the basis of aluminium oxide formation. The invention also concerns a device for detecting and analyzing fumes exiting from the furnace and an aluminium melting furnace comprising said detecting device.

TECHNICAL FIELD AND PRIOR ART

The invention relates to the field of aluminum smelting and to aluminummelting furnaces.

It applies especially to aluminum scrap recycling processes.

In the field of secondary aluminum smelting, the impossibility of beingable to measure and control many phenomena and parameters during meltingconstitutes an obstacle to understanding the melting process and also toobtaining better performance and to establishing rules for eliminatinglosses during melting.

The melting takes place in rotary or reverberatory furnaces. The processmay be continuous, but most furnaces work in batch mode. Charging withmaterials takes place via huge doors that open into the reverberatoryfurnaces and via the main door in rotary furnaces. To introduce largevolumes of scrap, the furnace must be charged two or three times percasting cycle.

Aluminum or its alloys must be melted above the melting point. However,in order for the liquid aluminum to flow suitably during the subsequenttreatment, for example in casting machines, it is necessary for themolten metal to reach a temperature level of 760° C. It is alsoendeavored to avoid any overheating of the melt pool, especially above780° C., at which temperature the rate of oxidation increasesconsiderably, almost exponentially.

During a casting cycle, there may firstly be distinguished the initialperiod, when the materials are solid, which allows the absorption of alarge amount of heat converted into the latent heat of melting ofaluminum at 660° C.

Next, when the metal is liquid, its thermal conductivity drops to halfits value in the solid state. Overheating may occur, but not uniformly,owing to heat distribution problems.

Large reverberatory furnaces are relatively more sensitive to this lackof uniformity. Heat is introduced from the surface of the liquid, butthe ratio of densities is such that stable stratification occurs—hotliquid is in the upper part of the melt pool and cool liquid in thelower part.

Reverberatory furnaces include additional devices, such as gas injectorsor pumps intended essentially to ensure uniformity of the melt pool andallowing a more effective melting procedure to take place.

Rotary furnaces employ only their intrinsic mixing principle. Heat istransmitted by heating the refractory that rotates beneath theliquid—the heat is introduced both via the upper surface exposed to theflame and via the bottom, which is in contact with the rotatingrefractory. Stratification is thus destabilized.

Metal losses during melting occur not only in the oxidized layer itself,but also because liquid is trapped in the oxidized skeleton. A trappedmixture of aluminum and alumina oxides then floats on the molten liquid.Trapped aluminum may represent 20 to 80% of the mass of what is called“dross”.

To obtain a good aluminum yield from recycled scrap requires losses onignition to be limited.

Two solutions are also applied in order to reduce metal losses in thedross.

Rotary furnaces, and also certain reverberatory furnaces, are firstlycharged with fluxes, these being based on salts (essentially a mixtureof NaCl and KCl), which are intended to reduce the wetting of the oxideskeletons by trapped aluminum.

The other solution is regular removal of dross from the surface. Notonly does the dross floating on the liquid metal represent a loss ofmetal, but it also constitutes a layer of insulating material coveringthe liquid. As this layer is formed from alumina, it has a high meltingpoint (greater than 1300° C.)—it does not melt further, but continues togrow. Introducing heat through such an insulating layer thereforerequires ever increasing power, that is to say the upper layer isoverheated. As the old saying of foundrymen goes, “dross generatesdross”. This dross may be removed by scraping the surface—thisconstitutes the dedrossing procedure.

This dedrossing operation is manual or in any case not highlymechanized.

It is essentially carried out, in reverberatory furnaces, by opening thedoors, thereby introducing a large amount of air into the furnace. Thishas two drawbacks, namely cooling (the return to the previoustemperature usually requires twice the time than that during which thedoor is open) and inflow of oxygen from the air, which easily oxidizesthe aluminum.

In addition, there exists no means for determining when the doors haveto be opened for dedrossing.

The yield of metal is determined by weighing the solid scrap at entryinto the furnace, with a deduction for the estimated weight of paint andcoatings, and by weighing the weight of metal obtained at exit from thefurnace. During the heating, such paint and coatings undergo pyrolysis,which is completed when the charge reaches 580° C.

However, these production techniques provide no indication about themoment when the dross forms and builds up during the casting cycle. As aconsequence, the execution of any action, such as limiting the timespent at high temperature in order to reduce the production of thisdross, relies on the operator's experience and on empirical rulesobtained laboriously over a long period of time. Any conclusion relatingto such an action can be drawn only on a statistical basis since thematerial of the scrap, by its very nature, varies in its origin and itsquality.

In reverberatory furnaces, the most advanced aluminum producers usecertain regulating methods, such as verification of the temperaturelevel of the refractory by a thermocouple, which reduces the power ofthe burners or which causes one or more burners to be shut down, when acritical temperature level is reached. However, these operations areessentially aimed at protecting the refractory.

They do not constitute direct indication about the formation of hotspots, where oxidation takes place.

Other producers check the temperature of the liquid metal by dipping athermocouple into the metal. When a temperature above 760° C. isdetected, the power of the burners is reduced or the furnace is chargedwith fresh materials.

However, this is merely a local indication, and hot spots may occur atother places.

Monitoring the temperature level of the refractory or of the liquidmetal is therefore not sufficient for these operations.

Moreover, all these solutions can be implemented on reverberatoryfurnaces, whether fixed or tilting, but they are not suitable for rotaryfurnaces.

Other solutions have been presented in the literature, but these areaimed in particular at avoiding oxidation by preventing the surface ofthe metal from coming into contact with any oxidizer.

Thus, document JP 58-227706 proposes using measurement of the CO and H₂contents in the flue gases to ensure that, in a furnace for meltingnonferrous metals, the fitted burners operate in substoichiometric modewithin a range of values of the oxidizer flow rate/fuel flow rate ratiogoing from 95 to 100%.

Document EP 962 540 discloses a combustion technique for melting a metalin a furnace. According to this technique, an oxygen-containing gas issent into the furnace so that it remains separated from the metal meltpool by the flame of the burner.

The burner then operates substoichiometrically. The influx of oxygencomes from the zone located above the flame, the latter forming a screenbetween the oxygen of the gas and the surface of the molten metal.

Document U.S. Pat. No. 5,563,903 describes a technique whereby an inertor nonoxidizing gas forms a screen between the surface of the moltenmetal (aluminum) and a combustion zone located in the upper part of thefurnace.

Document U.S. Pat. No. 3,759,702 relates to a technique in which themelting takes place initially in the open air, with a mobile burnerabove the surface of the materials to be melted. The flame of the burneris reducing, owing to the fact that, in the burner feed, there is aslight stoichiometric excess of fuel relative to oxygen.

These techniques are either complicated to implement or unsuitable formelting in a furnace.

The problem therefore arises of finding a technique for obtaining a highmetal yield in a melting furnace.

The problem also arises of how to detect the precise moment whenoxidation of the metal takes place during a casting cycle.

Knowing when oxidation occurs would make it easier to regulate thecombustion operations in order to reduce or control this oxidation,without it being necessary to use production statistics that reflect asmuch the quality of the scrap as the melting techniques themselves.

Moreover, knowing the moment when oxidation occurs and the amount ofoxide formed would make it easier to decide when to implement thededrossing operation.

SUMMARY OF THE INVENTION

The invention firstly relates to a method of detecting the formation ofaluminum oxides on the surface of an aluminum melt pool in an aluminummelting furnace, characterized in that the variations in COconcentration and in the temperature in the flue gases exiting thefurnace are detected.

The invention also relates to a method of melting aluminum, in which:

-   -   solid aluminum is introduced into a furnace;    -   the aluminum is melted, in order to form an aluminum melt pool;    -   the variations in carbon monoxide (CO) concentration and the        temperature in the flue gases exiting the furnace are detected;    -   the formation of aluminum oxides on the surface of the aluminum        melt pool is deduced therefrom; and    -   the melting process is regulated depending on the formation of        aluminum oxides.

While aluminum is undergoing oxidation in the furnace, the CO andtemperature variations are in the same direction, or have the same sign,which allows this oxidation to be easily detected.

According to the invention, it is therefore possible to detect positivevalues of the product, or of the ratio, of the CO and temperaturederivatives over time so as to identify the appearance of aluminumoxidation, and to do so without having to open the furnace for visualobservation and without an operator having to intervene.

It is therefore possible to use the analysis of the gases given offduring melting to detect the formation of oxides, this being achievedusing the temperature and CO concentration signals.

Optionally, the variations in CO₂ and/or O₂ and/or H₂O may also beobserved.

In particular, the inflows of air into the furnace according to thechange in H₂O concentration may be evaluated.

The analysis of the furnace flue gases may be used as a basis forregulating a melting process by:

-   -   adapting the combustion power density;    -   or adjusting the ratio of the flow rate of oxidizer to the flow        rate of fuel feeding the burners;    -   or stirring the melt pool by means of pumps or by injecting        liquid;    -   or an indication of the fact that the level of combustion power        above the stirred melt pool may or may not be increased.

Such flue gas analysis may also be used to optimize the frequency ofdedrossing.

A temperature measurement, or a sampling operation conditional on thetemperature, may furthermore be applied so as to distinguish between thesignal variations due to oxidation and those due to pyrolysis of thepaint and coatings formed on the aluminum scrap or fragments.

The invention also relates to a device for detecting the formation ofaluminum oxides on the surface of an aluminum melt pool in an aluminummelting furnace, characterized in that it comprises means of detectingthe CO concentration and temperature variations in the flue gasesexiting the furnace.

In an alternative way of implementing the invention, the formation ofaluminum oxides on the surface of an aluminum melt pool in an aluminummelting furnace is detected using means for detecting the variation inhydrogen concentration of the atmosphere above the surface of the meltpool and means for measuring the temperature in the flue gases exitingthe furnace.

Such a device may furthermore include means for detecting variations inCO₂ and/or H₂O and/or O₂ concentration.

Means may be provided for comparing the directions of variation in theCO concentration and in the temperature and/or for taking the product orthe ratio of the temporal variations in the CO concentration and in thetemperature. Means may also be provided for comparing the values of themeasured CO and/or H₂ contents with the respective CO and/or H₂ contentsthat the burner would have produced in the absence of interaction withthe charge. Such values may be established by calibrating the burner.

The invention also relates to an aluminum melting furnace, of thereverberatory or rotary type, which includes the devices and/or means asexplained above.

BRIEF DESCRIPTION OF THE FIGURES

FIGS. 1, 4 and 5 show various embodiments of an aluminum melting furnaceaccording to the invention.

FIGS. 2, 3 and 6 show gas concentration measurements made at the exit oftwo furnaces.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

According to the invention, variations in carbon monoxide (CO)concentration in the flue gases exiting the furnace are detected and thetemperature variations in the flue gases are measured.

FIG. 1 shows schematically a furnace 2 in which an aluminum melt pool 4is obtained by melting aluminum scrap. A burner 24, fed with an oxidizerfluid (for example oxygen) and a combustible fluid (for example anatural gas), produces a flame 11 for reaching the desired temperaturein the furnace.

Several burners may also be employed.

The references 15 and 17 furthermore denote oxidizer feed means and fuelfeed means, respectively.

Means 10 for measuring the variations in the amount or concentration ofCO are placed at the exit of the furnace, more precisely in a flue 13for discharging the flue gases 6.

Means 12 also allow the temperature variations to be measured atapproximately the same place.

There may also be means 14, 16, 18 for measuring the water vapor (H₂O)and/or carbon dioxide (CO₂) and/or oxygen (O₂) concentration at the exitof the furnace, in the same flue duct.

According to one illustrative example, the measurement means 10, 14, 16,18 may comprise one or more diode lasers, whereas the means 12 maycomprise an independent thermocouple.

FIG. 2 shows various curves I-V indicating, respectively, during amelting process in a dry-hearth furnace:

-   -   Curve I: the variation in carbon monoxide (CO) content;    -   Curve II: the variation in temperature;    -   Curve III: the variation in carbon dioxide (CO₂) content;    -   Curve IV: the variation in oxygen (O₂) content;    -   Curve V: the variation in water vapor (H₂O) content.

Curves I and II show a correlation between the CO variations and thetemperature variations.

FIG. 3A and 3B show, respectively, during a melting process in a rotaryfurnace:

-   -   Curve I′: the variation in CO content;    -   Curve II′: the variation in temperature;    -   Curve III′: the variation in CO₂ content;    -   Curve IV′: the variation in O₂ content.

FIG. 3A corresponds to the case of a high-temperature furnace and FIG.3B to the case of a low-temperature furnace.

The conclusions allowing these measurements to be interpreted are givenbelow. Reference will also be made to Table I below.

The composition of the combustion flue gases, essentially consisting ofcarbon dioxide CO₂ and water vapor H₂O, is modified by the oxidationprocess. Oxidation occurs not only by the action of pure oxygen or ofoxygen contained in air, but also owing to the presence of CO₂ and watervapor H₂O.

In other words, oxygen (O₂), carbon dioxide (CO₂) and water vapor (H₂O)may interact with the aluminum charge, forming alumina.

In fact, the mass of dross produced after deducting the trapped aluminumcannot be explained by direct oxidation alone. The amount of oxygen fedin excess to the burner, increased by the air introduced into thefurnace, is not sufficient for this purpose. TABLE I Mode of Mode ofVariation in O₂-rich O₂-lean Sampling H₂O combustion combustionconditions Simple Depends only No CO (dCO/dt)/(dT/ com- on the intakeobserved dt) is negative bustion of air (CO/CO₂) is case known for agiven lack of O₂ Flue- Below the 1^(st) sub- (dCO/dt)/(dt/dt) T > 400°C. gas/ aforementioned case: is positive (increased charge reference noCO (CO/CO₂) is from 100 to interaction value for the observed greaterthan 400° C. same flow CO₂ is above for (furnace- meter settings stablefixed flow specific Increased in conditions value)) temperature 2^(nd)sub- case: CO observed

In Table I, the modes of combustion rich in oxygen (O₂) and lean inoxygen correspond to a burner reference signal established with respectto an empty furnace or with respect to cold aluminum. An evaluationbased on the settings of the flow meters may be used as reference.

Simple combustion refers to a combustion system with no interaction withthe charge (either the aluminum furnace is empty or the aluminum iscold) but with a certain air intake into the furnace. This may in allcases be hybrid combustion with air and oxygen. The water vapor (H₂O)concentration is then a measure of the air intake.

It may be assumed, in order to demonstrate this, that the charged scrapcontains no water. Explosions due to water and to aluminum are a problemthroughout the aluminum industry and the charge is stripped of any waterand even moisture.

The water vapor (H₂O) concentration is then diluted by the air intakeand may be expressed as:air intake flow rate=theoretical flue gas flow rate[theoretical(H₂O)−measured (H₂O)/measured (H₂O)]including in the case of hybrid combustion with air and oxygen; the H₂Oconcentration is a measurement of the air intake if it is compared withthe H₂O content determined theoretically from knowledge of the fuel flowrate.

Given that the amount of oxygen available is determined by addition of:

-   -   the oxygen introduced by the air intake;    -   the oxygen measured by the flow meters (pure oxygen and added        air in the hybrid or enriched combustion mode),        the water vapor concentration therefore allows the amount of        oxygen available for combustion to be determined.

When interaction with the charge occurs, the water vapor concentrationdecreases according to the equation:2Al+3H₂O=Al₂O₃+3H₂,which is accompanied by the evolution of hydrogen. This phenomenon mustbe taken into account when evaluating the gaseous compositions on whatare called “dry” flue gases, i.e. after the removal of water vapor,usually by condensation. Interaction between aluminum and water vaporincreases the flow rate of dry flue gases.

The carbon monoxide (CO), carbon dioxide (CO₂) and temperature (T)signals in simple combustion systems can be explained as follows.

In oxygen-lean systems, carbon monoxide (CO) forms and the carbondioxide (CO₂) content decreases. This is imposed by the mass constraintapplied to the carbon atoms.

The CO/CO₂ ratio is independent of the air/oxygen combustion process. Itdepends only on the lack of oxygen.

The energy released in oxygen-lean systems is reduced. It is known thatthe maximum combustion efficiency occurs for a maximum CO₂ production.The signal indicates that the ratio of the CO and T variations over time(or the ratio of the derivatives with respect to time (dCO/dt)/(dT/dt))is negative (the CO concentration increases and this increase results inpoor combustion, T therefore decreasing).

When interaction with the charge occurs, and in apparent oxygen-richconditions, either CO production is observed or a temperature increase,at constant CO₂, may be observed.

More precisely, when CO is given off by the reaction:Al+3CO₂=Al₂O₃+3CO,it burns with the addition of heat to the system (T therefore increases)according to the equation:3CO+3/2O₂=3CO₂.The CO combustion conditions are possibly not effective because oflimited mixing of the reactants (insufficient intensity of turbulence).

The addition of heat due to the combustion process is enhanced by theexothermic nature of the reaction with aluminum. The signal is thereforesuch that (dCO/dt)/(dT/dt) is positive. However, it should be noted thatmost aluminum producers tend to work under oxygen-lean conditions.Therefore Co is produced from two sources, namely incomplete combustionand reaction with alumina Al₂O₃.

The CO produced by the burner has a defined value for a known oxygenshortage. The production of CO by oxidation is deduced from the total COmeasurement less this amount produced by the burner. In addition, duringan aluminum melting phase, the burner settings vary slightly. The COvariations associated with oxidation are similar to the variations inthe total amount of CO.

When interaction with the charge occurs, under oxygen-lean conditions, apositive (dCO/dt)/(dT/dt) value is a clear indication of oxidation,since the reaction with alumina is exothermic.

For the same oxygen-lean setting, that is to say of the same fuel andoxidizer injection flow rate settings, (CO)/(CO₂) increases more quicklyin the case of simple combustion. The increase in the CO/CO₂ ratioconfirms the detection of oxidation as this ratio is independent of thehydrogen (H₂) content, even on dry flue gases.

It is preferable to subject a positive value of (dCO/dt)/(dT/dt) to ameasurement of the temperature or to sampling conditional on thetemperature.

At low temperature, some of the carbon is released from the charge bypyrolysis of the paints and coatings on the aluminum scrap, carbonmonoxide (CO) then coming both from the actual combustion and from thedecomposition of carbon dioxide (CO₂).

If the aluminum scrap has been pretreated, this pyrolysis does not takeplace in the furnace.

Pyrolysis takes place up to a temperature of about 580° C. At lowertemperatures, CO may appear whereas oxygen may be lacking and thetemperature increases.

The (dCO/dt)/(dT/dt) ratio is then positive. In fact, in such a case thecombustion process is of the type:fuel+solid carbon particles+oxygen=combustion products.

If the oxygen feed conditions remain unchanged with respect to simplecombustion of the fuel, there is a lack of oxygen.

The process of pyrolyzing the paint and coatings appears to be toochaotic in the aluminum furnace for it to be followed or observed.

Consequently, a positive value of (dCO/dt)/(dT/dt) is preferablysubjected to a condition on the temperature.

The latter may be measured in the liquid metal, but preferably in theflue gases, the temperature of the flue gases reflecting the temperatureof the metal. A temperature difference of 100 to 400° C. allows heattransfer.

The temperature measurement may be compared with a predeterminedthreshold value, for example in the computer 20.

It should also be noted that, because of this correlation between theflue gas temperature and the metal temperature, it is possible for eachfurnace to be subjected beforehand to a learning period, the furnacesencountered in the aluminum industry varying widely.

In addition, the various modes of heat transfer do not lend themselvesto an easy solution by modeling, and a learning procedure, for examplebased on neural networks, is possible, but not to the exclusion of asimpler measurement of the temperature difference between the metal andthe flue gases.

Other conditions may in practice result in a (dCO/dt)/(dT/dt) signalwith a positive value.

This is the case when the burners are operating on hot metal, which mayoccur for example after the burners have been shut down for a shortperiod as a result of overheating of the refractories. In this case, theincrease in temperature of the flue gases is not a reflection of adisequilibrium in the combustion or a chemical interaction with thecharge, but simply an influx of energy.

However, if carbon monoxide (CO) is produced at the same time, thisindicates that aluminum oxide is produced on the hot liquid metal, veryprobably beneath the burner.

Next, when the oxidized layer covers the liquid, the oxidation slowsdown and the carbon monoxide (CO) content decreases. A positive value of(dCO/dt)/(dT/dt) is then again an oxidation detector.

An alternative method of implementing the invention consists indetecting the oxidation on the aluminum surface by a direct measurementof the CO content of the flue gases from which is subtracted the COcontent that the burner would have produced under similar temperatureand oxidizer/fuel injection conditions in the absence of interactionwith the charge.

Practical implementation involves a device at any point similar to thatshown in FIG. 1 but with the addition, to the oxidizer injection means15, of a flowmeter 21 and, to the fuel injection means 17, of anotherflowmeter 23. This arrangement is shown in FIG. 4.

Initial calibration of the burner is used to determine the CO contentproduced by the burner alone, by recording the CO content for variousinjection flow rates and flue gas temperature values. Such CO valuesmeasured by the device 10 are tabulated and input into the computer 20.

These tabulated values are subtracted from the contents measured by 10during melting of the aluminum and under the same fuel flow rate,oxidizer flow rate and flue gas temperature conditions. The value thusobtained indicates the content of CO produced by the oxidation reactionbetween aluminum and CO₂ according to the reaction:Al+3CO₂=Al₂O₃+3CO.

For reasons similar to those described above, it is necessary tovalidate this oxidation detection by temperature-conditional samplingsince the pyrolysis products given off at low temperature make the tableof CO values invalid.

A third method of implementing the invention is obtained by measuringthe H₂ content of the flue gases from which is subtracted the H₂ contentthat the burner would have produced under similar temperature andoxidizer/fuel injection conditions in the absence of interaction withthe charge.

Practical implementation involves a device at any point similar to thatshown in FIG. 4, but with the addition, to the flue gas compositiondetection means, of a means 19 for measuring the H₂ content. Thisarrangement is shown in FIG. 5.

Initial calibration of the burner is used to determine the H₂ contentproduced by the burner alone, by recording the H₂ content for variousinjection flow rates and flue gas temperature values. Such H₂ valuesmeasured by the device 10 are tabulated and input into the computer 20.

These tabulated values are subtracted from the contents measured by 10during melting of the aluminum and under the same fuel flow rate,oxidizer flow rate and flue gas temperature conditions. The value thusobtained indicates the content of H₂ produced by the oxidation reactionbetween aluminum and H₂O according to the reaction:2Al+3H₂O=Al₂O₃+3H₂.

For reasons similar to those described above, it is necessary tovalidate this oxidation detection by temperature-conditional samplingsince the pyrolysis products given off at low temperature make the tableof CO values invalid.

It is preferred to measure CO rather than H₂ because the H₂ content ismore difficult to measure, since hydrogen (H₂) is soluble in liquidaluminum, which may falsify the measurement. However, since the area forexchange between the atmosphere and the liquid metal is limited, thedetection of oxidation by H₂ is possible since the H₂ measurement isclear cut as FIGS. 6 a, 6 b and 6 c show.

These FIGS. 6 a, 6 b and 6 c show time recordings of the CO and H₂concentrations during melting in a furnace fitted with a burneroperating in combustion mode with oxygen. In the figures, curves I showthe CO contents and curves II the H₂ contents. FIG. 6 a corresponds to aflue gas temperature of 935° C. and FIGS. 6 b and 6 c correspond to atemperature of 1080° C.

The above analysis of the flue gases may be used as a basis forregulating an aluminum melting process.

After solid aluminum has been introduced into a furnace, it is melted,during which operation the carbon monoxide and temperature variations inthe flue gases exiting the furnace are detected.

The signals output by the sensor 10, 12, and optionally 14, 16 and 18,may be subjected to appropriate signal processing and then used asinputs for a regulating procedure. All these operations may be carriedout by means of a computer 20.

One possible regulation is that of reducing the power density of theburner 24 when oxidation is detected, the computer 20 giving, forexample, a setpoint value to the feed means 15 and 17.

Another possibility is to modify the ratio of the flow rates of fuel andoxidizer delivered to the burner 24, here again by acting on the means15 and 17. Usually, this composition is adjusted either to give themaximum power, with a slight oxygen excess (oxygen content in the fluegases between 0 and 2 to 3% oxygen), or by using, in certain producerplants, only burners operating with an oxygen-lean mixture, thusensuring the absence of direct access of oxygen to the aluminum. Such acombustion gas atmosphere with a reducing character is obtained eitherby partly closing off the oxidizer injection device 15 or, if possible,by opening the fuel injection device 17 further. From an oxidizer flowrate/fuel flow rate ratio of 100%, it is possible to lower this ratio to70%.

Covering the metals with a reducing atmosphere only after oxidation hasbeen detected makes it possible to achieve savings as regards thermalefficiency when no combustion losses exist.

For this purpose, means (not shown in FIG. 1) may be provided forintroducing such a reducing gas into the furnace, for example a supplyof this gas and a duct for introducing this gas into the furnace, theopening of the duct being controlled by the computer 20 when oxidationis detected.

In fact, an oxygen-lean flame produces carbon monoxide (CO) containingsome energy that is not transmitted to the charge. Highly efficientoxyburners may tolerate such a loss of energy.

Another method of regulation consists, when the onset of oxidation hasbeen detected, in stirring the melt pool 4 using pumps 30 controlled bythe computer 20.

In this case, the flue gas analysis may also indicate whether the powerlevel of combustion above the stirred melt pool can be increased or hasto be reduced.

Moreover, the flue gas analysis according to the invention and theaddition of the amount of oxide produced over time can be used as adecision-making tool as regards the dedrossing operation.

Optimization of the dedrossing frequency is an advantage from thestandpoint of thermal efficiency (no cooling inside the furnace untilthe door is opened) and as regards the yield of metal (since unnecessaryaccess of air to the hot metal is avoided).

Finally, the analysis according to the invention, and in particular thatof the water vapor (H₂O) concentration, can be used as a furnacediagnostic.

An air intake diagnostic is important both for thermal efficiency andfor evaluating thermal equilibrium.

The invention is applicable to the aluminum smelting field. In thesecondary aluminum industry, a 1% improvement in the amount of metalproduced is equivalent to a 10% energy saving.

1-30. (canceled)
 31. A method of melting aluminum comprising: a)introducing a aluminum charge into a furnace with at least one burner;b) melting said aluminum to form an aluminum melt pool; c) detecting thevariations in the carbon monoxide (CO) concentration and the temperaturein the flue gases; d) deducing the formation of aluminum oxides on thesurface of said aluminum melt pool; and e) regulating said meltingprocess depending on said formation of said aluminum oxides.
 32. Themethod of claim 31, further comprising detecting said variation or saidformation of at least one member selected from the group consisting of:a) water vapor (H₂O); b) carbon dioxide (CO₂); and c) oxygen (O₂). 33.The method of claim 31, further comprising comparing the directions ofsaid variations in said carbon monoxide (CO) concentration and saidtemperature.
 34. The method of claim 31, further comprising calculatingthe ratio or the product of said variations in said carbon monoxide (CO)concentration and said temperature.
 35. The method of claim 31, furthercomprising: a) measuring the fuel flow rate and the oxidizer flow rate;b) determining the ratio of said oxidizer flow rate to said fuel flowrate; and c) calculating the theoretical carbon monoxide (CO) content ofsaid flue gases that said burner would have produced without anyinteraction with said aluminum charge.
 36. The method of claim 35,further comprising: a) measuring the carbon monoxide (CO) content insaid flue gases; b) subtracting the value of said theoretical carbonmonoxide (CO) content from said measured carbon monoxide (CO) content;and c) deducing the amount of said aluminum oxide present.
 37. Themethod of claim 31, further comprising: a) measuring the fuel flow rateand the oxidizer flow rate; b) determining the ratio of said oxidizerflow rate to said fuel flow rate; and c) calculating the theoreticalhydrogen (H₂) content of said flue gases that said burner would haveproduced without any interaction with said charge.
 38. The method ofclaim 37, further comprising: a) measuring the hydrogen (H₂) content insaid flue gases; b) subtracting the value of said theoretical hydrogen(H₂) content from said measured hydrogen (H₂) content; and c) deducingthe amount of said aluminum oxide present.
 39. The method of claim 31,further comprising determining whether the temperature of said melt poolis greater than the temperature for pyrolizing any coatings and paint onsaid aluminum.
 40. The method of claim 31, further comprising measuringthe temperature of said flue gas.
 41. The method of claim 31, furthercomprising a means for detecting at least one member selected from thegroup consisting of: a) carbon monoxide (CO); b) water vapor (H₂O); c)carbon dioxide (CO₂); and d) oxygen (O₂).
 42. The method of claim 41,wherein said means comprises at least one diode laser.
 43. The method ofclaim 31, further comprising reducing the power of said at least oneburner after detecting formation of said aluminum oxide.
 44. The methodof claim 31, further comprising modifying the stoichiometry of saidburner after detecting formation of said aluminum oxide.
 45. The methodof claim 31, further comprising stirring said aluminum melt pool afterdetecting formation of said aluminum oxide.
 46. The method of claim 31,further comprising dedrossing said surface of said aluminum melt poolafter detecting formation of said aluminum oxide.
 47. The method ofclaim 31, further comprising introducing a reducing atmosphere into saidfurnace after detecting formation of said aluminum oxide.
 48. Anapparatus which may be used for detecting the formation of aluminumoxides on the surface of an aluminum melt pool in an aluminum meltingfurnace, comprising a means of detecting in the flue gases exiting saidfurnace at least one member selected from the group consisting of: a)carbon monoxide (CO) concentration; b) hydrogen (H₂) concentration; andc) temperature variations.
 49. The apparatus of claim 48, furthercomprising a means for detecting variations in at least one memberselected from the group consisting of: a) carbon dioxide (CO₂); b) watervapor (H₂O); and c) oxygen (O₂).
 50. The apparatus of claim 48, furthercomprising a means for comparing the directions of variation in thecarbon monoxide (CO) concentration and in said temperature.
 51. Theapparatus of claim 48, further comprising a means for calculating theratio or the product of said variations in said carbon monoxide (CO)concentration and in said temperature.
 52. The apparatus of claim 48,further comprising a means for detecting if said carbon monoxide (CO)content exceeds the value of the (CO) content from said burner asrecorded during the prior calibration of said burner.
 53. The apparatusof claim 48, further comprising a means for detecting if said hydrogen(H₂) content exceeds the value of the (H₂) content from said burner asrecorded during the prior calibration of said burner.
 54. The apparatusof claim 48, further comprising a means for detecting at least onemember selected from the group consisting of: a) carbon monoxide (CO);b) water vapor (H₂O); c) carbon dioxide (CO₂); and d) oxygen (O₂). 55.The apparatus of claim 54, wherein said means comprises at least onediode laser.
 56. The apparatus of claim 48, further comprising a meansfor reducing the power of said burner, following detection of saidaluminum oxide formation.
 57. The apparatus of claim 48, furthercomprising a means for modifying the oxidizer flow rate/fuel flow rateratio of said burner.
 58. The apparatus of claim 57, wherein said ratiois modified in the range of about 70% to about 100%.
 59. The apparatusof claim 48, further comprising a means for stirring said aluminum meltpool, following a means for detection of said aluminum oxide formation.60. The apparatus of claim 48, further comprising a means for dedrossingsaid surface of said aluminum melt pool.
 61. The apparatus of claim 48,further comprising a means for introducing a reducing atmosphere intosaid furnace.
 62. An apparatus which may be used as an aluminum meltingfurnace comprising a means of detecting in the flue gases exiting saidfurnace at least one member selected from the group consisting of: a)carbon monoxide (CO) concentration; b) hydrogen (H₂) concentration; andc) temperature variations.
 63. The apparatus of claim 62, wherein saidfurnace is either a rotary or reverberatory type.